Mentioned in a few songbook threads and in bits and pieces elsewhere:

...It was a very ringing song, though not so grandly jubilant as the next, which was really impressive as the singers pealed it out, when marching or rowing or embarking.

V. MY ARMY CROSS OVER.

"My army cross over,
My army cross over.
O, Pharaoh's army drownded!
My army cross over.

"We'll cross de mighty river
My army cross over;
We'll cross de river Jordan,
My army cross over;
We'll cross de danger water,
My army cross over
We'll cross de mighty Myo,
My army cross over. (Thrice.)
O, Pharaoh's army drownded!
My army cross over."

I could get no explanation of the "mighty Myo," except that one of the old men thought it meant the river of death. Perhaps it is an African word. In the Cameroon dialect, "Mawa" signifies "to die."

Higginson, T.W., Negro Spirituals, Atlantic Monthly, June 1867